There have been put to practical use and widely used photosensitive lithographic printing plates (PS plates: presensitized printing plates) produced by subjecting the surface of a sheet of a metal (such as aluminum) to grinding, anodization, hydrophilic treatment and so on to produce a substrate and forming a photosensitive layer of a photosensitive diazo resin, photopolymerizable composition or photo-crosslinkable composition on the substrate. Such PS plates are subjected to imagewise exposure and development to thereby remove non-image areas of the photosensitive layer, and then printing is conducted by utilizing the hydrophilicity of the substrate surface in non-image areas and the lipophilicity of the photosensitive layer in image areas.
However, a metal sheet having hydrophilic treatment subjected thereto is disadvantageous in that it is expensive, that the handling of the sheet is restricted owing to the poor flexibility thereof, and that the downsizing of the plate-making equipment is difficult, though the sheet is excellent in strengths necessary for the substrate of a printing plate and in retention of hydrophilicity in printing to attain high durability Further, there have also been known plates produced by forming a hydrophilic resin layer on an inexpensive and flexible substrate such as waterproof paper, plastic film or synthetic paper and forming a photosensitive layer on the hydrophilic resin layer. However, the resin constituting the hydrophilic layer is poor in water resistance owing to its hydrophilicity and therefore is liable to be swollen with a developing solution used in development or with dampening water used in printing. Therefore, printing with such a plate results in poor durability and resolution and suffers from peeling or scumming disadvantageously.
In order to improve the water resistance of the hydrophilic resin, there have been proposed processes of adding a crosslinking agent such as glyoxal, melamine, urea or amino resin to the resin and heating the resulting composition to cure it (see JP-A-51-104902, JP-A-53-141702 and JP-A-57-69097) However, it is very difficult to form a hydrophilic resin layer excellent in both hydrophilicity and water resistance by such a process, because sufficient crosslinking of a hydrophilic resin with importance attached to water resistance gives an insufficiently hydrophilic layer leading to scumming, while slight crosslinking thereof with importance attached to hydrophilicity gives an insufficiently water-resistant layer which is swollen in development or printing to result in poor resolution and to cause scumming. Further, the hydrophilic resin layer is poor in adhesion to a hydrophobic photosensitive layer, so that image missing is liable to occur after development or during printing.
Meanwhile, there have been disclosed processes of forming on a substrate a layer containing a resin (such as maleic anhydride copolymer) which is originally hydrophobic but is converted into a hydrophilic resin during or after alkali development conducted after exposure (see JP-A-57-98395, JP-A-60-90338 and JP-A-60-125699). Such a maleic anhydride copolymer is less hydrophilic than the above hydrophilic resins and therefore is easily applicable on hydrophobic substrates such as plastic films. However, a plate comprising a hydrophilic resin layer containing the maleic anhydride copolymer is swollen in development or printing owing to the poor water resistance of the layer that is made by development to come to the surface in non-image areas, which is causative of poor resolution and scumming. Further, there have been proposed the formation of a layer from a combination of a polyamide resin with a urea resin and a pigment (see JP-A-55-40406) and the formation of a layer from a composition comprising a water-soluble acrylic copolymer and a crosslinking agent (see JP-A-58-14794). However, both layers are poor in adhesion to a photosensitive layer, particularly when the photosensitive layer is a photopolymerizable one which is generally extremely hydrophobic.